'He will continue to fight and will appeal the verdict,' his wife said.

Vietnamese human rights lawyer and activist Nguyen Van Dai was jailed for 15 years on Thursday, his wife said, along with five other activists who were given prison terms of 7-12 years.

Dai, 48, was charged in a Hanoi court with activities "aimed at overthrowing the people's administration" at a tightly guarded trial in the communist country.

"I'm very disappointed with the trial and strongly protest it," Dai's wife Vu Minh Khanh said. "He is innocent and he pleaded innocent at the trial. He will continue to fight and will appeal the verdict".

But foreign ministry spokeswoman Le Thi Thu Hang told reporters that "In Vietnam, there is no such thing as a 'prisoner of conscience', and there's no such thing as people being arrested for 'freely expressing opinion'," Hang said.

Tight control

Despite sweeping reforms to its economy and showing openness to social change, Vietnam's ruling Communist Party retains tight media censorship and has zero tolerance for criticism.

In March 2013, Dai and others formed the "Brotherhood for Democracy" which conducted anti-government activities to "build multi-party democracy" in Vietnam, according to a copy of the official indictment against Dai seen by the Reuters news agency.

"The Vietnamese government should thank them for their efforts to improve the country instead of arresting and putting them on trial," Human Rights Watch Asia director Brad Adams said.

Dai had been awaiting trial since his arrest in December 2015. He has previously served four out of five years of a prison term he received in 2007 for "anti-state propaganda".

Hundreds of police were posted outside the court during Thursday's trial and a planned march by families of the detained activists was stopped.

Hanoi-based activist Nguyen Chi Tuyen said police arrived at his house as the trial began and followed him for the entire day.

"They followed me to my office, invited me to coffee, waited outside my office, followed me back home, and now they're waiting outside my house again," said Tuyen, who added that police had put glue in the lock of the house of a fellow activist to stop him going out.